This supplemental proposal outlines progress toward the high resolution crystal structures of two unusual electron transport proteins from Azotobacter vinelandii, a nitrogen fixing, aerobic bacterium. Ferredoxin 1 (Fd1) crystallizes in two forms: tetragonal with one molecule per asymmetric unit (Z equals 1) and triclinic (Z equals 4). The tetragonal form is a new crystal form not previously described which diffracts X-rays to 2.0 A resolution. Triclinic crystals diffract to 2.5 A. Cytochrome c5 crystallizes in two forms: triclinic with Z equals 2 (1.7 A resolution) and monoclinic with Z equals 1 (2.0 A resolution). Both tetragonal Fd1 and triclinic c5 crystals are ideally suited for diffractometer data collection. The initial objective is to obtain 3-4 A resolution structures of each protein. Azotobacter Fd 1 is the first well characterized member of a new class of iron-sulfur proteins, not like other bacterial ferredoxins. Its two (Fe4S4S4cys) clusters differ by over 0.7 v in reduction potential. One cluster has novel EPR and redox properties analogous to one of the centers in nitrogenase. Unlike other cytochromes c, c5 forms a stable dimer. As one of two abundant diheme cytochromes in Azotobacter, c5 transfers electrons from cytochrome b1 to cytochrome O.